Many different types of systems have been proposed for locating targets, e.g., buildings, armored vehicles, aircraft on the ground, etc., from surrounding clutter, e.g., trees, bodies of water, roads, etc. The systems have included optical recognition techniques, infrared detectors, radar systems, and other types of devices. The effectiveness of each of the systems is affected in different ways by ambient conditions. For example, darkness or inclement weather can render an optical system marginally effective or totally useless in locating targets. Heat detector systems lack the ability to discriminate between actual targets and diversionary warm bodies. Radar systems compatible with small aperture seekers are oftentimes not capable of sufficiently discriminating between targets and surrounding clutter. Thus, there is a continuing need for an improved system to locate potential targets or landmarks located in a cluttered environment.
Within the radar art, it is well known that a polarized signal incident upon clutter can become depolarized on reflection. Accordingly, prior art techniques have considered the use of polarization information to enhance target detection and clutter discrimination by means of radar.
For example, in U.S. Pat. Nos. 4,323,898 and 4,323,899, polarization radar detectors are proposed wherein signals reflected from targets illuminated with quasi-monochromatic electromagnetic signals such as frequency modulated continuous wave (FMCW) radar are received by antennae responsive to orthogonally polarized components of the reflected signals. These received orthogonal signal components are modified such that one component is phase shifted with respect to the other signal component and the amplitudes of the signal component are varied according to predetermined angles of polarization. The resultant signals are further analyzed in a processor which detects a polarization of the reflected signals by estimating the Stokes parameters for the reflected signal. The Stokes parameters are then used to generate detection criteria that are compared to a predetermined threshold level.
Another polarization radar device is described in U.S. Pat. No. 3,945,005 which describes a radar system which includes a periodically pulsed transmitter adapted to generate linearly polarized energy, a duplexer, a conventional receiver, a directional antenna system provided with a rotatable dipole antenna, and a linear polarization rotator interconnecting the duplexer and the dipole antenna. The pulsed transmitter and the dipole antenna are rotated in synchronism and a cathode ray tube indicator is provided for generating a circular trace of the reflected polarized energy.
The systems and techniques described in these three patents as well as the other devices and methods known in the prior art require precise knowledge of the transmitter and/or reflected energies' phase reationships to determine a complete set of the scattering properties of the objects being observed. This results in complicated hardware and data processing system requirements which have caused the prior art systems to be only moderately successful.